The present invention relates to an optical module, and more particularly, to an optical module in which a light-emitting element and an optical function element formed with an optical waveguide are mounted in one casing.
Optical function elements whose substrates are constructed using lithium niobate (LiNbO3), lithium tantalate (LiTaO3), or the like, are used as modulators in optical communications for use mainly in high-speed regions exceeding a transmission rate of 10 Gbits/s. The optical function elements using the foregoing materials have been mounted in hermetically sealed casings each separate from the casing of a semiconductor laser diode element. This form of mounting has been mainly due to the difference in thermal expansion coefficient (a) between the optical function element and the semiconductor laser diode element.
Luminous elements represented by semiconductor laser diode elements have hitherto been mounted in packages formed of materials whose thermal expansion coefficients are much the same as those of optical function elements. Output light from these light-emitting elements is fiber-emitted via condensing lenses.
Similarly, the optical function elements formed with an optical waveguide having an electro-optical effect have also been mounted in packages formed of materials almost equivalent to the optical function elements in terms of thermal expansion coefficient. The light-incident/exit sections of such an optical function element are coupled to fibers via lenses or directly fiber-coupled using a bonding agent or the like.
Polarization-maintaining fibers are used to admit fiber-introduced light into the optical function element that has polarization direction dependence. To make the light from a light-emitting element enter the optical function element, the polarization-maintaining fiber inside the light-emitting element package and the polarization-maintaining fiber at the input side of the optical function element package have been coupled to each other using connectors or splices.
Japanese Patent Laid-open No. 11-218648 describes an optical composite module having a light-emitting element and an optical function element housed in welded respective independent packages. This optical composite module does not need to have a plane-of-polarization preserving fiber between the light-emitting element and the optical function element.
Also, Japanese Patent Laid-open No. 2001-272572 describes a method for mounting an optical function element module of a structure in which the input end and output end of an optical function element are fixed to a fiber and this fiber is further fixed to a package.
As mentioned above, a space for connecting fibers is needed to connect a module that has a mounted a light-emitting element such as a laser diode, and a module with a mounted optical function element such as an optical modulator. Also, an expensive polarization-maintaining fiber and polarization direction matching are required in order to connect both modules while maintaining the plane of polarization. Therefore, partly to reduce dimensions by reducing space requirements, and to reduce costs by dispensing with a polarization-maintaining fiber, it is necessary to mount both the light-emitting element and the optical function element in one package.
However, mounting both the light-emitting element and the optical function element in one casing (package) made of a material whose thermal expansion coefficient significantly differs from those of the two elements will deteriorate reliability and characteristics of the elements.